On The Holy Trinity by St. Augustine
A kind of image of the Trinity is pointed out, even in the outer man; first of all, in those things which are perceived from without, viz. in the bodily object that is seen, and in the form that is impressed by it upon the sight of the seer, and in the purpose of the will that combines the two; although these three are neither mutually equal, nor of one substance. Next, a kind of trinity, in three somewhats of one substance, is observed to exist in the mind itself, as it were introduced there from those things that are perceived from without; viz. the image of the bodily object which is in the memory, and the impression formed therefrom when the mind's eye of the thinker is turned to it, and the purpose of the will combining both. And this latter trinity is also said to pertain to the outer man, in that it is introduced into the mind from bodily objects, which are perceived from without.